The combined cost of military operations in Afghanistan and Iraq over the past 12 months has almost doubled to more than £3bn, a cross-party group of MPs revealed yesterday.

The costs of operations by British forces in Afghanistan has risen to more than £1.6bn, a year-on-year increase of 122%. More surprisingly, given the reduction in troops in Iraq, the cost of Britain's military presence there has also increased to £1.6bn, a year-on-year rise of 72%, the Commons defence committee said.

The costs are about 50% more than the government forecast three months ago, the report said. It came as military officials made it clear that the number of British troops in Iraq would not now be cut this spring to the number previously indicated by Gordon Brown.

The total cost of operations in Iraq and Afghanistan since the invasion of Iraq in 2003 now totals about £10bn, according to estimates based on annual official figures from the Treasury and Ministry of Defence. The sharpest increases were for buying, repairing and replacing new armoured vehicles and other equipment acquired under a special Urgent Operation Requirements system, referred to as UORs. The report said these costs in Iraq and Afghanistan had risen "far beyond the scale of other costs". It added: "The MoD needs to make clearer the reasons for these considerable increases."

James Arbuthnot, the committee's chairman and a former Tory defence minister, said: "Few people will object to the investment being made in better facilities and equipment for our troops in Iraq and Afghanistan. However, this estimate represents a lot of public money. The MoD needs to provide better information about what it is all being spent on." The figures were seized on by Liam Fox, the shadow defence secretary, who said: "We have been complaining for a long time that these operations are under-resourced. It appears that this is something the government seems to belatedly recognise."

Nick Harvey, Liberal Democrat defence spokesman, said the report "clearly shows how the Iraq war is continuing to bleed our finances dry, leaving soldiers in Afghanistan overstretched and under-equipped".

Kate Hudson, the chair of CND, said: "The human costs of the wars in Iraq and Afghanistan are clear, with an estimated 655,000 dead in Iraq alone, but the opportunities lost by spending these billions on further destruction rather than on humanitarian reconstruction adds to the long list of tragedies unleashed by Bush's wars."

Bob Ainsworth, the armed forces minister, said: "I am quite clear what we are spending it on and because this money comes from the Treasury reserve, spending it in one theatre does not come at the expense of the other. We are spending it on better equipment ... and paying our forces more, including a tax-free operational bonus - in both Iraq and Afghanistan."

The committee observed that the cost of operations in Iraq was as high as in Afghanistan. This was despite the reduction in British troops in Basra. But the MoD has been forced to invest in equipment for troops as attacks by insurgents increase.

In a statement yesterday, the MoD avoided any reference to Brown's October announcement that he planned to reduce the number of UK troops in Basra to 2,500 from the spring of this year. The MoD said only that it intended "to reduce troop numbers in Iraq over the coming months". A senior British defence official said Iraqi generals in Basra wanted a significant British presence as they built up their forces. "Where are we going to be at the end of 2008? We don't know," the official said.